Curved nose punch



July 23, 1968 H. E. lHLE 3,393,550

CURVED NOSE PUNCH Filed Aug. 16, 1966 INVENTOR. HANS E. IHLE Agent United States Patent 3,393,550 CURVED NOSE PUNCH Hans E. Ihle, Smyrna, Ga., assignor to Lockheed Aircraft Corporation, Burbank, Calif. Filed Aug. 16, 1966, Ser. No. 572,856 2 Claims. (Cl. 72333) ABSTRACT UP THE DISCLOSURE A punch is provided to pierce aluminum and soft or mild steel in such a way that the fatigue life of the material around the hole at stress levels of 15,000 to 40,000 p.s.i. is as high as and up to 54% higher than that where the hole is produced by drilling. This punch has a curved end which is the arc of a circle having a radius of to 20 times the thickness of the material to be pierced. This are extends from the side of the punch curving inwardly a distance equal to to of the thickness of the material to be pierced terminating in a sharp edge defining a flat punch face. The center of the circle is located on a line intersecting the point where the arc begins and extending perpendicular to the punch side.

This invention relates to punches employed to pierce holes in metal and more particularly to a specially designed, curved nose punch by which structural metal members having a shear strength up to approximately 60,000 psi. may be pierced in a single operation without loss of fatigue life as compared with drilled or routed holes.

While not limited to, the present invention finds special utility in airframe manufacture where interal structural members are often required to be pierced with lightening, access and/or passage holes. Conventional present day punches have not been employed to produce these holes because of the resultant loss of fatigue life to the member and more elaborate operations, such as drilling, routing, reaming, etc., have been required.

The objection to the use of conventional punches resides in the fact that the break clearance, i.e., the distance between the edge of the punch and the adjacent edge of the die opening when the parts are brought together, induces concentrated deformation of the metal during severance which results in a tearing thereof during the final portion of the penetration. Thus, a conventional punch with a sharp cutting edge pierces a hole in the sheet metal workpiece by shearing the metal about /2 to /3 of its thickness and then during the balance of the penetration simply breaking the metal or slug out of the hole and into the die. The ultimate wall of such a hole consists of a shear and a break line with the trans-verse dimension of the hole in the break line area greater than that in the shear area.

In many cases drilling, reaming, and/ or routing is objectionable, usually in that it is not compatible with high speed production, since it requires power equipment with awkward-to-handle power lines, individual cutting operations, etc., all of which are relatively slow, time-consuming, and expensive. In an effort to avoid this, it has heretot'ore been proposed to punch undersized holes in the workpiece and subsequently enlarge such holes by one or more additional punching operations. In brief, this amounts to shaving the workpiece by a series of cutting operations until the desired dimensions are obtained. This practice is equally objectionable and often results in some waste when the workpiece is inadvertently damaged.

The present invention is therefore directed to improvements in the metal punching art and proposes a novel punch having a critically formed cutting and working nose whereby holes having precise dimensions and defined ice by work-hardened edges may be produced in metal in a single punching operation. This cutting and Working nose is characterized by its curved marginal end surface having a radius and deflection determined by and in proportion to the thickness of the material to be punched. Effectively, the curved nose punch cold works the metal workpiece simultaneously with the cutting operation and eliminates a break clearance as well as the usual break line.

The cold working of the metal as determined by the radius of the curved nose of the punch is critical in that overworking can produce an objectionable swelling or head on the metal at and around the ultimate hole and distortion of the metal workpiece or part. At the same time, it is important that the hole-defining surface of the metal is work-hardened to the maximum so as to approximate as near as possible the fatigue life of a drilled or reamed hole.

With the above and other objects in view as will be apparent, this invention consists in the construction, combination and arrangement of parts all as hereinafter more fully described, claimed, and illustrated in the accompanying drawing wherein a fragment of a punch constructed in accordance with the teachings hereof is illustrated in association with a metal workpiece and the companion die to show the geometry of the curved nose and. the manner in which such configuration is produced.

Referring more particularly to the drawing, 20 designates the shank of a punch having an overall diameter D substantially equal to that of an opening 0 in a complementary die 21 whereby coaction of the punch and die produces a hole in a workpiece 22 of thickness T. At its outer end, the shank 20 is formed with a curved portion 23 commencing at a point S in the length of the shank 20 substantially equivalent to the standard punch length and terminating in a punch face 24 disposed in a plane perpendicular to the longitudinal center line 25 of the shank 20.

The curve, i.e., the deflection H of the shank surface, is within the range of 0.02 and 0.20 and preferably about 0.15 of the thickness T of the workpiece 22 and is established by an arc of a curve having a radius R that is approximately times the deflection H. The center C of this circle is located by drawing a line from the point S perpendicular to the center line 25 and locating the radius R on this line. The length of the curved portion 23 is established by an intersection I formed where the circle about the center C crosses a line 26 defining deflection H. The punch face 24 is then established, extending transversely from the intersection I.

In order to substantiate the results to be obtained through the use of the present curved nose punch, tests were conducted to compare specimens containing holes produced by different methods. In all other respects, the specimens were identical being machined from the web of a 7075-T6 aluminum alloy extrusion. Each specimen measured 16 inches long, 3 inches wide at its ends with a narrow 1 inch central portion, and 0.153 inch thick. In the central portion of each specimen a hole having a diameter of 0.250 inch was produced by either drilling, reaming, by use of a standard punch or a curved nose punch as herein proposed. The holes of the curved nose punch specimens were of two types, one in which the deflection H was selected at 0.02T and the other where H was 0.03T.

Additionally, some of the specimens were worked by surface shot peening in order to determine the effect thereof when combined with the different hole punching methods. This shot peening was accomplished using number 23 stainless steel shot to produce an intensity range of 0.006 to 0.008 Almen. No special attention was given the hole during shot peening.

Fatigue tests were conducted at two constant ampli- 3 tude stress levels, viz., maximum stresses of 30,000 and 40,000 p.s.i. In each case, the stress was based on net area and the stress ratio was held constant at +0.l. Five specimens prepared per each of the hole cutting methods were tested at each stress level for a total of ten specimens per method.

Axial-load, tuning-fork-type fatigue machines were used in all tests. These machines have a static capacity of 15,- 000 pounds and a dynamic capacity of il0,000 pounds at a cycling rate of 1,5 to 2,200 cycles per minute. Specimen mean load vvas applied with a hydraulic actuator while a sinusoidal alternating load was applied by a variable eccentric mass in the form of a wheel attached to one arm of the machine. The eccentric wheel was driven by a variable-speed electric motor employing an automatic speed control device. Desired dynamic load was obtained by changing the rotational speed of the eccentric mass.

A conventional mechanical counter attached to the shaft of the eccentric wheel recorded the number of cycles applied. An automatic cut-off device stopped the machine upon failure of the specimen. A strain-gaged transducer placed in series with the test specimen was used to sense the applied load. The transducer output was monitored with an analyzer comprised of calibrated balancing potentiometers, a three-kilocycle carrier amplifier and an oscilloscope. The desired load was set on a calibrated potentiometer and the carrier wave was amplitude modulated by the transducer signal. The oscilloscope was used only as a null balance indicator. Maximum, minimum and mean loads were monitored by this system. The complete load analyzing system was calibrated using a 20,000 pound capacity load cell which in turn was calibrated using a universal testing machine calibrated by proving rings certified by the National Bureau of Standards. This system provided an accuracy of 13%.

The results of these tests are set forth in Table I below to facilitate a comparison of the hole preparation methods.

mens, shot peening either decreased or did not appreciably change the fatigue life.

It was, therefore, concluded that a higher fatigue life can be expected from a piece of structural metal having a hole pierced by a curved nose punch than from one in which a hole has been pierced by a conventional punch. Moreover, this metal with a curved nose punch hole is superior in fatigue life to one with a drilled hole as presently employed and may be readily substituted therefor in many applications, such as, for example, high speed production of certain classes of metals, to the end that a more efficient and economical practice may be had. Where warranted, shot peening may be employed in some cases to enhance the mean log life of parts pierced by the curved nose punch as herein proposed, whereas such cold Working of parts drilled, reamed or punched in the usual manner adds nothing to their fatigue life.

In separate tests, similar specimens were prepared in which holes were pierced using punches having substantially greater arcuate deflections H. From these tests it was learned that the fatigue life of specimens became longer as the arcuate deflection H was increased. As this increase reached 0.15T, a practical limit was reached in that subsequent stripping became excessively difiicult requiring additional procedures which detracted from the advantages gained by using a curved nose punch.

What is claimed is:

1. A curved nose punch adapted to coact with an opening in a die in cutting a hole in a metal workpiece comprisinng a shank having a diameter approximately equal to that of said die opening, and an arcuate outer end extending from said shank and terminating in a substantially perpendicular punch face defined 'by a peripheral cutting edge, said arcuate end having a deflection within the range of 0.02 and 0.20 of the thickness of the workpiece and a radius of approximately 100 times said deflection.

TABLE I.(THOUSANDS OF CYCLES TO FAILURE Maximum N ct Stress=40,000 p.s.i.

Maximum Net Strcss=30,000 p.s.1.

Type of Hole Specimen Number lvlllean Specimen Number Biean 0g 0g 1 2 3 4 5 Life 6 7 8 9 10 Lite Drill 16 16 17 19 17 17. 0 63 49 46 44 50. 0 Ream 23 29 14 18 23 20. 8 309 350 199 890 733 426. 1 Ream, SP 14 16 17 17 17 16.2 53 63 60 55 57. 6 Standard Punch"... 11 11 10 11 9 10.4 26 35 37 48 35. 5 Standard Punch, SP 9 8 9 9 8 8.6 34 29 32 32 33 32.0 Curved Nose Punch (H=0.02T) 18 18 15 13 18 16. 3 39 43 40 65 49 46.3 Curved Nose Punch (H=0.02T) SP. 17 13 14 17 14 14. 9 41? 46 46 37 72 47. 5 Curved Nose Punch (I-I=0.03T) 18 19 22 22 20. 2 5b 69 44 45 34 48.2 Curved Nose Punch (H=0.03'I) SP. 24 22 24 17 25 22. 2 87 89 64 76 53 72. 4

1 Test was considered invalid. SP but Peened.

All of the specimens tested failed through the hole section and visual examination of the failed specimens revealed fatigue origins at the edges of the holes. As is evident from Table I, the specimens with holes produced by the standard punch displayed the lowest fatigue life at both stress levels. Specimens which were punched with the curved nose punch having a deflection H of 0.03T and especially those subsequently shot peened exhibited a fatigue life that compared favorably with those that were reamed at the higher stress level and appreciably better than those of the drilled hole specimens at both stress levels. Except for the curved nose punch speci- 2. The punch of claim 1 wherein said deflection is approximately 0.15 of the thickness of the workpiece.

References Cited UNITED STATES PATENTS WILLIAM W. DYER, JR., Primary Examiner. G. WEIDENFELD, Assistant Examiner. 

